There are many well known methods of coupling a laser light into an optical fiber. In the case of semiconductor laser diodes the simplest solution is the so called “butt coupling” consisting in mechanical fixing of a fiber tip close to a laser mirror, disclosed for example in U.S. Pat. No. 4,837,768, U.S. Pat. No. 4,807,956 and JP 60123077 A.
Improved optical coupling is achieved by focusing laser light into a fiber tip using lenses. Collimated laser beam can be simply focused into the optical fiber using a single lens. Light emitted by a laser diode is focused using several cylindrical and spherical lenses, which has been disclosed, among others, in the U.S. Pat. No. 6,768,593. Laser light can also be focused on a fiber tip by means of a spherical mirror using a method disclosed in U.S. Pat. No. 4,902,093.
There are also known methods for coupling laser light from a few sources into the one, common optical fiber. For example, for two lasers, two collimated laser beams with perpendicular polarizations can be superimposed using the polarizing beamsplitter. In case when more than two laser sources are necessary it is possible to use a method known from WO 2005/041262. In this solution each source is coupled into a separate fiber, for example using one of the methods described above, and then a bundle of the fibers is coupled into one bigger fiber.
In order to obtain high power of light in an optic fiber it is common to use a so called laser bar (array) or stacked laser bars. A laser bar consists of a set of many identical parallel laser diodes (for example 20 diodes 0.5 mm apart) mounted in a single package and driven by the same voltage. A beam emitted by such bar is slowly divergent in the plane of the bar (slow axis) and strongly divergent is the perpendicular direction (fast axis). Therefore, first a cylindrical lens is used for collimating laser light in the perpendicular direction and next some optical setup is used for focusing light into the fiber. The examples of such methods are disclosed, among others, in U.S. Pat. No. 5,333,077. U.S. Pat. No. 6,700,709, U.S. Pat. No. 5,825,551, and U.S. Pat. No. 6,504,650 and U.S. Pat. No. 6,765,725. A simpler solution for a laser bar is a cylindrical lens coupling light from each laser in the bar into a separate thin fiber, and then the bundle of these fibers can be coupled to a single thick fiber, as it has been disclosed for example in EP 0486175. An interesting solution is a conical fiber tip (tapered light guide) with a diffraction grating diffracting a beam emitted by a laser bar disclosed in EP 1429434. U.S. Pat. No. 5,617,492 discloses a solution concerning stack of laser bars consisting in collimating a beam of each laser of the stack and then coupling all the light beams into a fiber using a single lens. European patent description EP 7964S2 discloses a method of coupling into a fiber a laser beams from a few bars not forming an array. In case of laser bars and stacks all lasers are similar (produced in the same epitaxial process) and mounted in a common package. A power supply is also common for all lasers.
There are also known solutions for coupling laser beams from many separate sources, with separate power supplies and emitting light with different wavelengths. U.S. Pat. No. 4,828,357 discloses focusing beams from many lasers into one spot using a set of mirrors. Each laser beam is directed by two individual mirrors and then one concave mirror focuses all beams into one spot. However, this publication does not include any teaching concerning adjustments of the elements of such complex optical system. Practice proves, that with the increased number of lasers such system will be more difficult to adjust and more sensitive to mechanical shocks.
The prior art closest to the present invention has been disclosed in international publication WO-A1-00/45208. An apparatus A method disclosed in the publication uses a single reflecting element consisting of a set of individual reflecting-surfaces. The laser beam from each source passes through the first optical system of two afocal lenses and then is reflected from one of the reflecting surfaces towards the second optical system, focusing light on a fiber facet. In the disclosed solution the second optical system also consists of two lenses. The role of the afocal systems is to reduce a diameter of the beams incident on the mirrors and on the fiber facet. Examples of embodiments disclosed in this publication consider only flat configurations i.e. such that the beams from all laser sources lie in one plane from the sources to the fiber. Moreover, this publication does not contain any teaching concerning a method of adjusting all components of the system, which is crucial for the practical realization of the solution. JP-A-2007/065600 and JP-A-403001881 disclose apparatus for coupling laser light derived form several laser sources in to one optic fiber in which the laser sources surround a set of individual reflecting surfaces jointly forming a lateral surface of a regular pyramid. Number of sides of the pyramid base equal to the number of the laser sources.